Theoretical studies of the mechanism of ethylene polymerization reaction catalyzed by diimine-M(II) (M = Ni,Pd and Pt) and Ti- and Zr-chelating alkoxides

We have analyzed the computational results for several elementary reactions of the ethylene polymerization process catalyzed by an alternative (to the existing metallocene catalysts) “non-cyclopentadienyl” catalysts such as diimine-M(II) (where M = Ni and Pd) and chelating bridged Ti- and Zr-complexes. The obtained data have been compared with those for the existing zirconocene-based catalysts. In general, it was shown that: (i) the resting stage of the process is a metal-olefin-alkyl complex, the olefin coordination energy of which is a few kcal/mol larger for diimine-M(II) systems than zirconocene or dialkoxide systems; (ii) the rate-determining barrier is a migratory insertion barrier calculated from the metal-olefin-alkyl complex, which is found to be a few kcal/mol larger for the diimine-M(II) system compared to the Cp₂ZrCH ₃ ⁺ catalyst. The presence of certain flexible bridging ligands X in the Ti-alkoxide complex, [Y(Ph)X(Ph)Y]TiCH ₃ ⁺ , which are capable of donating electron density to the cationic metal center at various stages during the reaction makes this barrier a few kcal/mol smaller for the dialkoxide than the Cp₂ZrCH ₃ ⁺ catalyst. It was shown that an increase in the metal-bridge interaction decreases the migratory insertion barrier and, consequently, increases the catalytic activity of these complexes. Although the diimine-M(II) catalysts are less active than zirconocene-based ones, the microstructure of the polymers produced by the former catalyst, which is found to be a function of temperature, ethylene, steric bulkiness of the auxiliary ligands, and transition metal center, makes them attractive for practice. We also have studied the mechanisms of several chain termination/transfer reactions, as well as the role of steric effects in the studied elementary reactions. We have clearly demonstrated tremendous possibilities of the computational chemistry in solving complex problems of the homogenous catalyst, and its high capability of predicting new and more active catalysts for different commercially important processes including olefin polymerization reactions. This revised version was published online in June 2006 with corrections to the Cover Date.